Phenols are defined as organic compounds possessing one common structural feature—one or more aromatic rings having a minimum of one hydroxyl substituent per ring. The smallest phenol chemically is p-hydroxy benzene once widely used as an antiseptic but still available in some retail stores as a mildly disinfectant soap. Svobodova et al (Biomed Papers, 2003 147(2):137-145) have published an excellent review of natural phenols, phenolic acids and flavonoids derived from plants and the role they play in the prevention of UV-induced skin damage in humans. One plant-derived phenol currently undergoing extensive testing in the medical arena is trans-resveratrol or trihydroxy stilbene isolated from red grapes. Naturally occurring phenolic acids contain two distinguishing features: the hydroxycinnamic and hydroxybenzoic acid groups. Typical examples are caffeic and ferulic acids which have been proven effective in protecting human skin from UVB-induced erythema. Flavonoids are polyphenolic compounds that are ubiquitous in nature and are categorized, according to chemical structure into flavonols, flavones, flavanones, isoflavones, catechins, anthocyanidins and chalcones. Over 4,000 flavonoids have been identified and have aroused considerable interest recently because of their potential beneficial effects on human health such as antiviral, anti-allergic, anti-platelet, anti-inflammatory, antipruritic, anti-microbial, anti-tumor and antioxidant activities. One natural polyphenol that has been extensively tested in both animals and humans is a catechin from green tea leaves called epigallocatechin gallate (EGCG) which is used as an example for this disclosure.
Epigallocatechin Gallate and Solar Radiation:
The many benefits of topical application of green tea and its catechin components are summarized in a June 2005 issue of Life Extension Magazine entitled “Why sunscreens do not fully prevent skin cancer” (www.lef.org/magazine/magazine). The summary cites scientific articles showing that EGCG provides broad-spectrum protection by preventing several pathological changes associated with sun damage: inflammation, oxidative stress, DNA damage and immune system deficits. In one such article, Katiyar et al (Carcinogenesis, 2001 22(2):287-294) investigated the effects of topical application of EGCG to human skin on UV-induced markers of oxidative stress. Using immunochemistry on punch biopsies, the authors found that application of EGCG before a single UV exposure (4× minimal erythema dose or MED) markedly decreased UV-induced production of hydrogen peroxide, nitric oxide and lipid peroxide in both epidermis and dermis. EGCG pretreatment also inhibited UV-induced infiltration of inflammatory leukocytes into the skin. Leukocytes are considered to be the major producers of reactive oxygen species (ROS) which can cause a depletion of immune system cells. The in vitro ROS scavenging activity of EGCG was examined by Nanjo et al (Bioscience Biotechnology Biochemistry, 1999, 63(9):1621-1623) and found to be superior to other tea catechins in scavenging ROS such as superoxide anion and hydroxyl radicals—byproducts of hydrogen peroxide. Numerous studies have demonstrated that topically applied EGCG prevents these pathological changes and may help to prevent skin cancer. For example, experiments by Conney et al, (Proceedings National Academy Science USA 2002 99(19):12455-60) have demonstrated significant reductions in both tumor incidence and size following topical EGCG application and UV radiation versus controls in mice specially bred for their susceptibility to cancer.
Katiyar et al (Clinical Cancer Research, 200 6:3864-3869) investigated the impact of pre-treating human skin with green tea polyphenols prior to exposure of varying doses of UVB (0.5, 1.0, 2.0, 4.0×MED). The results demonstrated significant inhibition of UVB-induced erythema responses at all UVB exposures. The same authors utilized skin biopsies from volunteer buttocks to study the effect of single applications of GTP (0.4, 0.8, 1.2 and 1.6 mg/cm2 skin area) on UV-induced DNA damage in the form of cyclobutane pyrimidine dimers (CPD)—premutagenic lesions and the primary cause of melanoma. The results showed a significant dose-dependent decrease in CPD formation in both epidermis and dermis.
Of special interest are studies by Eun et al (The FASEB Journal, 2003 17(13):1913-article 10.1096/fj.02-0914fje. Published on line Aug. 1, 2003) that examined the effect of EGCG application under occlusion during a 6-week period to the buttocks of elderly healthy volunteers. Vehicle or EGCG-treated skin samples were obtained by punch biopsy. The EGCG application significantly stimulated the proliferation of structurally-supportive skin cells known as keratinocytes versus vehicle-treated samples along with a corresponding significant increase in epidermal thickness. In vitro experiments by the same authors demonstrated a dose-dependent stimulatory effect of EGCG on cultured keratinocytes and conversely, an inhibitory action against cultured squamous carcinoma cells, i.e., apoptosis. Of particular significance in these findings is the fact that two hallmarks of aging skin are both reduced skin thickness and keratinocyte cell growth and EGCG application has apparently reversed these trends. The anti-aging potential of EGCG was further demonstrated by these authors in a group of young volunteers. EGCG or vehicle control were applied to the buttocks during 48 hours and then treated with simulated solar radiation (2 MED). The protective action of EGCG against UV radiation was shown in skin biopsy results where a 4-fold greater increase in keratinocyte cell numbers were found in EGCG-treated samples versus vehicle-treated counterparts.
The inhibitory effect of EGCG on UVB-induced activation of nuclear factor kappa beta (NE-κB) in normal human epidermal keratinocytes (NHEK) was demonstrated by Mukhtar et al (Oncogene 2003 22:1035-440). NF-κB is a protein complex that is sequestered in the cytoplasm of almost all cell types by inhibitory protein kappa B (IκB) and is involved in cellular responses to stimuli such as stress, free radicals, ultraviolet irradiation and bacterial antigens. Once stimulated into action, NFκB sets off a cascade of proinflammatory cytokines such as tumor necrosis factor alpha (TNFα), interferon gamma (INF-γ), interleukin-1 and -8 (IL-1, IL-8) with implications in cancer, inflammatory diseases, septic shock, viral infection, and improper immune development. In the above-mentioned study the authors show that pretreatment of the keratinocytes to EGCG before UVB exposure resulted in a significant dose- and time-dependent inhibition of UVB-induced activation of NFκB. The results suggest that EGCG has the potential to protect against the adverse effects of UVB radiation via modulation in the NFκB pathway, thus protecting cellular targets against UVB-induced damage. The authors provide a molecular basis for the photo-chemopreventive effect of EGCG and suggest that green tea may be a useful agent against UVB-induced damage for human skin.
Epigallocatechin Gallate and Eczema
Eczema is a chronic inflammatory disease characterized by extreme itchiness, skin dryness, redness and inflammation initiated and exacerbated by stress and certain environmental factors such as extreme weather changes and allergens. Cutaneous infection, especially by Staphylococcus aureus and certain fungus such as Trichophyton rubrum and Malassezia spp, is known to be a contributing factor in eczema and often detected in the exzematous skin of patients. The antibacterial activity of whole and fractionated crude extracts of green tea (GTE) against various clinical isolates of pathogenic cutaneous bacteria was evaluated by Yam et al (FEMS Microbiology Letters 1997 152:169-74). The numbers of strains sensitive to GTE versus number strains tested were as follows: for S. aureus 33/33 (including 18 methicillin-resistant strains, MRSA), S. epidermidis (38/38) and Corynebacterium spp (2/2). The antibacterial activity of EGCG against 53 clinical isolates of MRSA was reported by Kono et al (The Journal of the Japanese Association of Infectious Diseases 1994 68(12):1518-22). The minimal inhibitory concentration (MIC) of EGCG was found to be 32-128 μg/ml against all strains. A time-kill study demonstrated that an isolate was bacteriostatic at 1-2 times the MIC and bactericidal at 6 times the MIC or ˜0.04% aqueous solution against MRSA. The in vitro antifungal activity of EGCG against clinical isolates of various dermatophytes was reported by Park et al (Yonsei Medical Journal 2011 52(3):535-538). The authors demonstrated that among the dermatophyte species tested T. rubrum was the most susceptible to EGCG—more susceptible than to flucytosine and similar to fluconazole. The MIC value for 11 clinical isolates of this fungus ranged between 1-16 μg per mL.
Most S. aureus strains from atopic lesions produce enterotoxins with superantigen properties that are capable of inducing skin inflammation. Hisano et al (Archives of Dermatological Research 2003 295:183-189) used one such superantigen staphylococcal enterotoxin B (SEB) injected intraperitoneally, with or without EGCG, to monitor proinflammatory or cytokine inhibition in BALB/c mice. Serum levels for the two proinflammatory agents TNF-α and INF-γ were significantly reduced by EGCG versus control. Noh et al (International Immunopharmacology 2008 8:1172-1182) used a mite allergen extract (Dermatophagoides pteronissinus) applied topically to the ears of NC/Nga mice to induce atopic dermatitis (AD) type skin lesions. This inbred mice strain maintained in non air-controlled environments spontaneously suffer from AD-like skin lesions with markedly elevated serum levels of immunoglobulin E (IgE). Topical application of EGCG daily, following the allergen extract treatment, significantly reduced ear swelling and significantly lessened clinical signs provoked by the mite allergen extract such as erythema, edema, scaling and excoriation when compared to vehicle only application.